1. Field of the Invention
The present invention relates to an anisotropically electroconductive adhesive material and to a connecting method.
2. Description of the Related Art
In recent years, bare IC chips which are furnished with gold or solder bumps as projecting electrodes have been directly flip-chip mounted on electrode pads of substrates to be IC-mounted or worked and mounted in chip size package (CSP) configurations. When chip mounting is carried out as such, an anisotropically electroconductive adhesive material in the form of a film, paste or liquid, in which an epoxy or other thermosetting resin and electroconductive particles have been compounded, is sandwiched between the projecting electrodes of the bare IC chip and the substrate to be IC-mounted, after which heat and pressure are applied thereto.
However, using gold bumps is problematic due to the extremely high cost of materials. Moreover, using solder bumps is problematic owing to the fact that when an electrolytic plating method is used to form very fine and uniform bumps, a resist step is required, and furthermore, before the solder plating is carried out, complex electrolytic plating steps are required, such as the forming of a underlying metal layer (Ti/Cu) and the forming of a metal multilayered plating barrier layer (Cu/Ni/Au).
Attempts have therefore been made to use nickel bumps, which are low in cost and which can be formed in a relatively simple electrolytic plating step.
However, the relatively high hardness of the nickel bumps when compared with gold or solder bumps means that plastic deformation occurs in the anisotropically electroconductive adhesive material because the electroconductive particles therein are crushed by the nickel bumps. This becomes problematic owing to the fact that a stable state of contact cannot be maintained between the nickel bumps of the bare IC chip and the electrode pads of the substrate to be IC-mounted, which results in diminished connection reliability.
With the foregoing problems with the prior art in view, it is an object of the present invention to provide an anisotropically electroconductive adhesive material with which it can be ensured that the same connection reliability observed in conventional anisotropically electroconductive connections which use gold or solder bumps is maintained, even when relatively hard bumps such as nickel bumps are used as projecting electrodes when effecting anisotropically electroconductive connections between electronic elements such as bare IC chips, which are furnished with projecting electrodes, and the connecting pads of wiring boards.
The present inventors perfected the present invention by discovering that the reliability of anisotropically electroconductive connections is intimately related to a 10% modulus of compressive elasticity (E) in the electroconductive particles in the anisotropically electroconductive adhesive material and the modulus of longitudinal elasticity (Exe2x80x2) of the projecting electrodes of the electronic element, and moreover that by adjusting the ratio of E to Exe2x80x2 over a specific range the connection reliability can be improved.
In other words, the present invention provides an anisotropically electroconductive adhesive material, comprising a thermosetting resin and electroconductive particles dispersed in the thermosetting resin, wherein a 10% modulus of compressive elasticity (E) in said electroconductive particles and a modulus of longitudinal elasticity (Exe2x80x2) of projecting electrodes of an electronic element to be connected by said anisotropically electroconductive adhesive material satisfy the below Relational Formula (1).
0.02xe2x89xa6E/Exe2x80x2xe2x89xa60.5xe2x80x83xe2x80x83(1)
Moreover, the present invention provides a connecting method, comprising sandwiching an anisotropically electroconductive adhesive material, in which electroconductive particles have been dispersed in a thermosetting resin, between projecting electrodes of an electronic element and the connecting pads of a wiring board, and applying heat and pressure thereto to make a connection therebetween while ensuring continuity between the electrical element and the wiring board, wherein, in the anisotropically electroconductive adhesive material, a 10% modulus of compressive elasticity (E) in the electroconductive particles and a modulus of longitudinal elasticity (Exe2x80x2) of the projecting electrodes of the electronic element that satisfy the Relational Formula (1) below.
0.02xe2x89xa6E/Exe2x80x2xe2x89xa60.5xe2x80x83xe2x80x83(1)